Multisampling: performance hit if disabled?

Just a quick question: if I request a pixel format with multisampling capabilities, but do not use them, will rendering to that pixel format give me a performance hit even if GL_MULTISAMPLE_ARB is disabled?

The reason I'm asking is because it would be very convenient to be able to request a multisampling pixel format in all cases, but only turn it on if I'm on the "high quality" rendering path.

I'm pretty sure once you get a multisample pixel format you'll see a performance hit regardless if GL_MULTISAMPLE_ARB is disabled (at least in my tests that's the case). You need to trash the context and set things up from scratch without multisampling to really "disable" it, however, there is an ATI-only extension to toggle FSAA at any time without mucking with pixel formats.